Structural biologists shed light on how a family of enzymes called TUTases regulate let-7, an essential regulator of development that is dyregulated in lung and kidney cancers, among others. The team used x-ray crystallography to capture the equivalent of freeze-frames of TUTases, at the resolution of individual atoms, interacting with other molecules to regulate the activity of let-7. this work will aid efforts to target TUTases as a way of increasing expression of let-7 in cancer cells.